Cobalt base alloy



v United States Patent (IOBALT BASE ALLOY Louis Prehn Jahnke, Madeira, Ohio, assignor to General Electric Company, a corporation of New York 'No Drawing. Application March 6, 1956, Serial No. 569,688

9 Claims. (Cl. 75-171) This invention relates to a cobalt base alloy which is particularly adapted for use in applications where great strength and formability at high temperature is required.

One of the objects of the invention is the provision of an alloy adapted for use as a bucket material in turbosuperchargers, gas turbines, or other jet propulsion apparatus.

A further object is the provision of an alloy with improved tensile ductility and impact strength with no reduction in the stress rupture strength at high temperatures.

In recent years, great strides have been made in the de- .velopment of cobalt base alloys for such devices as superchargers, gas turbines, and jet propulsion apparatus. These alloys are required to have certain physical characteristics at high temperatures, such as high strength against mechanical and thermal stresses and high tensile ductility and workability. An alloy fully meeting all these requirements has not been developed, and although several alloys have been proposed for use in high temperature applications, the utility of such alloys has been limited.

By way of illustration, a cobalt base alloy containing approximately 10% nickel has excellent rupture strengths but possesses very low tensile ductility and impact strength.

A specific example of such a prior art alloy is as follows:

Percent Carbon 0.4 Boron 0.4 Chromium 26.0 Tungsten 15.0 Nickel 10.0 Balance cobalt This alloy will be hereinafter referred to as alloy H.

My improved alloy contains about 0.10% to approximately 0.20% carbon, about 0.25% to approximately 0.55% boron, about 24% to approximately 28% chromium, about 14% to approximately 18% tungsten, less than 1% nickel, less than about 1% manganese, and less than about 1% silicon.

The low carbon content and the absence or substantial absence of nickel in my alloy are in every sense regarded as being critical to the achievement of the optimum properties. If its carbon content is increased over about 0.20%, as in alloy H, ductility suffers badly. If the nickel is present in amounts greater than 1%, the properties of my alloy are decreased considerably. For an example of the effect of nickel on the alloy, the following two alloys were melted and extensively tested:

It is noted that alloys D and E difiered only in a 10% 2,816,024 I 'atented Dec. 10, 1957 .nickel contento'f alloy D. The properties of these alloys are shown below:

1,700 F./l00 1,600sIm- .honr Rupture 1,200 F. Tenpact .(Un-

Strength, sile Elonganotched- .p. s. i. tion, percent Round Bar),

it.-lbs.

21, 000 6 to 7 8 23, 000 6 t0 7 11. 5

The above tests show that alloy E maintained 10% higher rupture .strengths with an indication of increased impact resistance. This is an appreciable improvement.

Increasing .carbon content 'over 0.20% has the same general effect as increasing boron from 0.25 to "0.55%. Both result in lowering the ductility and increasing the strength. However, the ductility strength balance achieved with 'varying boron is always found to be better than that *foundwith varying carbon. Thus, the alloy of my invention contains only a relatively small range of carbon anda-larger range of boron. The total carbon plus boron should'not exceed 0.75% for adequate ductility for engineering usages. Stated another way, if carbon plus boron exceed 0.75%, the brittle carbide-boride phase structure becomes continuous and great brittleness and thus general uselessness as an engineering material result.

Thus, by correlating the various ingredients in the amounts given and by keeping the carbon and nickel contents within the ranges indicated, excellent high temperature properties are obtained.

A preferred alloy of the present invention contains:

Percent Carbon 0.15 Boron 0.40 Tungsten 15.0 Chromium 26.0 Balance cobalt This alloy will be hereinafter referredto as alloy A.

In the following tables, my alloy A is compared with the prior art alloy H referred to above. These tables'show that at temperatures in the 1200 F. to 1800 F. range, my alloy A possesses a far superior degree of tensile ductility than alloy H. These tables also show that the rupture strength at 16501800 P. (which is the important range for turbine bucket usage) is not decreased but is increased and also that the impact strength is increased at the temperature ranges shown. By balancing the composition and thus increasing the ductility, I have eliminated the existence of a continuous brittle phase normally associated with low ductility materials.

Tensile ductility PERCENT ELONGATION Temp. Alloy H Alloy A Hour rupture strengths Alloy H Alloy A Temp. Stress, Stress,

p. s. i. p. s. 1

Impact strengths CHARPY V NOTGH Alloy H,

Temp- Ft.-Lbs.

The above tests thus clearly indicate unusual high temperature properties of my alloy. 'f 7 f .1

Other typical examples of alloys falling within the scope of my invention are:

C B Cr W N1 Mn Si C0 Bal. Bal.

Alloy B Alloy C Alloys B and C also possess good physical characteristics at high temperature. Alloy B will be more ductile and not as strong as alloy A, while alloy C will be less ductile and considerably stronger. The primary etfect is that obtained by varying the boron. The following table shows the results obtained from a number of tests of varying boron content. All other elements remained constant.

Hours Until Fracture At- 1,200 F.

Tensile Ducttlity (Percent cl.)

1,600 F. Round Bar Strength, tt.-lbs.

Percent Boron p. s. i.

Omaninvention and as many possible changes may be made in the embodiments set forth, it will be distinctly understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An alloy containing 0.10% to 0.20% carbon, 0.25% to 0.55% boron, 24% to 28% chromium, 14% to 18% tungsten, with the balance essentialy all cobalt.

2. An alloy containing 0.10% to 0.20% carbon, 0.25% to 0.55% boron, 24% to 28% chromium, 14% to 18% tungsten, less than 1% manganese, and less than 1% silicon, with the balance essentially all cobalt.

- 3. An alloy containing 0.10% to.0.20% carbon, 0.25 to 0.55% boron, 24% to 28% chromium, 14% to 18% tungsten, less than 1% manganese, less than 1% silicon, and less than 1% nickel, with the balance essentially all cobalt.

4. An alloy containing 0.15% carbon, 0.40% boron, 15% tungsten, 26% chromium, and the balance essentially all cobalt.

5. An alloy containing 0.15% carbon, 0.40% boron,

, 15% tungsten, 26% chromium, less than 1% manganese,

and less than 1% silicon, and the balance essentially all cobalt.

6. An alloy containing 0.10% carbon, 0.25% boron, 24% chromium, 14% tungsten, less than 1% nickel, 0.5% manganese, less than 1% silicon, and the balance essentially all cobalt.

7. An alloy containing 0.20% carbon, boron, 28% chormium, 18% tungsten, 1% nickel, 1% manganese, 1% silicon, and the balance essentially all cobalt.

8. An alloy containing 0.15% carbon, 0.35% boron, 26% chromium, 15 tungsten, less than 1% nickel, 0.8% manganese, 0.8% silicon, and the balance essentially all cobalt.

9. A nickel-free alloy containing 0.15% carbon, 0.40% boron, 15 tungsten, 26% chromium, and the balance cobalt and incidental impurities.

References Cited in the tile of this patent UNITED STATES PATENTS vFranks et al. July 4, 1950 

1. AN ALLOY CONTAINING 0.10% TO 0.20% CARBON, 0.25 % TO 0.55% BORON, 24% TO 28% CHROMIUM, 14% TO 18% TUNGSTEN, WITH THE BALANCE ESSENTIALLY ALL COBALT. 